Why does home button in the UV/Image editor zoom out a lot?
As far as I remember, it snaps to the same zoom ratios as you use the numpad for (1:1, 1:2, 1:4, 1:8...). This is probably to avoid any scaling artifacts since the Image Editor does not filter the images.

If you want to view the image fit to the borders better, try pressing F. However this will not set it to 1:1 if the image is smaller than the region.

Personally, I think there could be some more consistency in this regard, keeping the same hotkeys throughout editors would be nice.